After a decline in rates of infection over several decades, a disturbing increase in the incidence of tuberculosis (TB) is occurring. Because TB is highly contagious it poses a profound threat to public health. TB bacteria are easily passed from person to person in airborne droplets formed when a person with active TB sneezes or coughs.
Even more alarming has been the rise of multidrug-resistant tuberculosis (MDRTB). Prior to 1984, about 10% of TB bacteria isolated from patients in the United States were resistant to even a single antibacterial drug. In 1984, 52% of patients were infected with Mycobacterium tuberculosis (also referred to as tubercle bacilli) resistant to at least one drug, and 32% were resistant to one or more drugs. Outbreaks of MDRTB have been reported in 13 states. Ten percent of the recorded MDRTB cases to date have occurred in previously healthy people whose mortality rate--70 to 90%--has been nearly the same as that of immunosuppressed persons with MDRTB (Snider and Roper, 1992).
The United States Centers for Disease Control (CDC) has released preliminary results of a joint study with the New York State Health Department showing that cases of drug-resistant TB have more than doubled since 1984. CDC data from the first quarter of 1991 show that many of these drug-resistant strains are resistant to both of the frontline TB drugs, rifampin and isoniazid. Outbreaks of MDRTB have occurred in hospitals in Miami and New York City, as well as in the New York State prison system. In one hospital in New York City, the median interval between diagnosis of MDRTB and death was only four weeks. Additional clusters of MDRTB were reported to the CDC in 1990 and 1991 from Mississippi, Missouri, and Michigan.
There are five frontline drugs known to be highly effective against Mycobacterium tuberculosis and five second-line drugs that can be used when resistance to one or more of the frontline drugs is detected. Ironically, in the United States, until April 1992, there were shortages of antituberculosis drugs, some of which are crucially needed when resistance to the frontline drugs rifampin and isoniazid is present. These shortages had occurred because several pharmaceutical companies had ceased production of these drugs.
Because of its persistence in the body, the tubercle bacillus is a notoriously difficult pathogen to control. Although bacille Calmette-Guerin (BCG) vaccine protects against severe tuberculosis meningitis and disseminated TB in children, its efficacy against pulmonary TB in adults has varied widely in different parts of the world. Treatment of conventional TB is effective, but expensive, requiring daily treatment with multiple drugs for a minimum of six months. There is a common tendency among TB patients to stop taking their drugs when the drugs begin to have their beneficial effect or to take the medications only intermittently. When this happens, relapses are frequent and very often are caused by drug-resistant tubercle bacilli that have survived the initial course of treatment. The emergence of drug-resistant M. tuberculosis is in many ways an index of individual compliance with antituberculosis chemotherapy and of the inability of the health care infrastructure to ensure adequate treatment. Many public health agencies that once could play key roles in this process have had their budgets cut drastically in recent years and hence are unable to perform this crucial service.
MDRTB is extraordinarily difficult to treat, and a majority of patients do not respond to therapy. Total treatment costs for an individual with MDRTB can be as much as 10 times the cost of traditional treatment; the cost of the treatment drugs alone can be as much as 21 times as great.
The preferred treatment for classical TB consists of isoniazid, rifampin, and pyrazinamide. For patients whose tubercle bacilli are thought to be resistant to isoniazid, a fourth drug, ethambutol, is commonly added to the regimen until drug susceptibility results are known. Isolates of tubercle bacilli resistant to both isoniazid and rifampin, now representing about 20% in some cities, require specialized treatment with additional medications, which may include streptomycin and ciprofloxacin for almost two years.
The tubercle bacillus is a slow-growing organism. Three to six weeks are needed to grow the bacteria in the clinical laboratory, and an additional three to six weeks are needed to screen for antibiotic resistance. Such extended laboratory procedures can result in a delay in diagnosis, which means that patients with unrecognized drug-resistant TB may be treated ineffectively and remain infectious for a longer period. In HIV-positive individuals, MDRTB usually causes death within 4 to 16 weeks after being diagnosed, which is often before laboratory tests on drug susceptibility and resistance can be completed.
There is no evidence that mutation rates in M. tuberculosis organisms have increased or that increased virulence is to blame for the recent deadly outbreaks of TB. It is likely that drug-resistant forms of tuberculosis arose because of patient noncompliance with the 6- to 12-month regimen of antibiotics required to treat TB. Ineffective treatment regimens also play a role in the rising incidence of TB. To address noncompliance, some states with high TB rates are considering approaches to outreach, such as expanding directly observed therapy (DOT); others may reestablish inpatient facilities similar to the TB sanatoria of the first half of this century. Standard treatment regimens for TB have also been updated. Instead of taking two or three antibiotics, TB patients now take four. Still, as noted earlier, the current shortages of antituberculosis drugs in the United States have made even standard treatment difficult.
A series of nitroimidazo[2,1-b]oxazole derivates was described in Sehgal, K. et al., "Novel Nitroimidazo[2,1-b]oxazole Formation from Reaction of 2,4(5)-Dinitroimidazole with Oxiranes (1)," J. Heterocyuclic Chem. 16:1499-1500 (1979). Compounds of this type have the following general formula (I): ##STR3## These compounds were described as potential radiosensitizing agents for use in the radiotherapy of cancer (Agrawal, K. et al., "Potential Radiosensitizing Agents. Dinitroimidazole;" J. Med. Chem. 22(5):583-586 (1979); Sehgal, R. et al, "Potential Radiosensitizing Agents. 2. Synthesis and Biological Activity of Derivatives of Dinitroimidazole with Oxiranes," J. Med. Chem. 24:601-604 (1981). More recently, certain nitroimidazole compounds were reported to exhibit antimicrobial properties, including antitubercular activity (see, e.g., Nagarajan, K. et al., "Nitroimidazoles XXI. 2,3-dihydro-6-nitroimidazo [2.1-b] oxazoles with antitubercular activity," Eur. J. Med. Chem. 24:631-633 (1989). In addition, the compound of formula (I) in which R is ethyl (2-ethyl-5-nitro-2,3-dihydro[2,1-b]imidazo-oxazole, also known as Ceiby-Geigy CGI 17341) has recently been shown to exhibit activity against Mycobacterium tuberculosis (Ashtekar, D. et al., "In Vitro and In Vivo Activities of the Nitroimidazole CGI 17341 against Mycobacterium tuberculosis," Antimicrobial Agents and Chemotherapy, 37(2):183-186 (1993).
Pseudomembranous colitis (PMC) is a serious intestinal disease marked by severe colonic inflammation, diarrhea, abdominal cramps, and mucosal plaques or pseudomembranes. PMC is caused by the over production of toxigenic Clostridium difficile in the gut. C. difficile is a spore-forming anaerobe and is the major nosocomial pathogen of PMC. The over growth of C. difficile occurs when the bacterial flora of the GI tract has been modified due to extensive use of broad spectrum antibiotics. Two toxins, A and B, are produced by C. difficile. The toxins attack membranes or microfilaments of colon cells producing inflammation and necrosis. Toxin A causes intestinal hemorrhage and fluid secretion while toxin B is cytotoxic.
PMC as a subclass of diarrheal disease has become a frequent complication of antibiotic use. PMC normally appears 5-10 days after onset of antibiotic therapy. A watery diarrhea is the most common symptom, occurring in 90-95% of all PMC cases (Aronsson, B. et al., J. Infect. Dis. 151:476-481 (1985)). Severe cases of PMC can cause high fever, leukocytosis, dehydration, electrolyte imbalance, and death (see Clostridium difficle: Its role in Intestinal Disease. R. D. Rolfe and S. M. Finegold, Ed., Academic Press Inc., New York (1988), and R. Fekety, OAntibiotic-Associated Colitis. Mediguide to Infectious DiseaseO Vol. 4, pp. 1-7 (1984)).
Patients at greatest risk include the elderly, debilitated cancer patients, and patients undergoing abdominal surgery. Untreated C. difficile produces 10-20% mortality in elderly or chronically debilitated patients (Dosik, G. M. et al., Am. J. Med. 67:646-656 (1979)). Worldwide incidence of PMC is unknown due to the lack of appropriate studies. However, in industrialized countries, C. difficile is rapidly becoming the most common enteric bacterial pathogen after Campylobacter and Salmonella (Bartlett, J., see Clostridium difficle: Its role in Intestinal Disease. R. D. Rolfe and S. M. Finegold, Ed., Academic Press Inc., New York, pp. 1-13 (1988)).
Antibiotics most frequently used to treat PMC include vancomycin, metronidazole, and bacitracin. Vancomycin is a very expensive treatment, $100-$400 for a ten day course. Relapse rate after vancomycin therapy has been shown in experimental animals (Swannson, B. et. al., Antimicrobial Agents and Chemotherapy, 35:1108-1111 (1991) and Bartlett, J. G. et al., Clin. Infect. Dis. (S4) S265-72 (1994)). Due to the increase of vancomycin resistant bacteria, the use of vancomycin for C. difficile infections may be on the decline. Metronidazole is less effective than vancomycin, however, it is also less expensive. Metronidazole is orally absorbed and may expose patients to potential side effects that are associated with the drug (PHYSICIANS DESK REFERENCE, 48TH EDITION, 1994, pp. 1704-1706). Metronidazole has a relapse rate similar to vancomycin. Bacitracin is an antibiotic polypeptide and is commercially available as a mixture of nine peptides. It is also expensive and no convenient oral dosage form is available.
Organisms of the genus Cryptosporidium are small obligate intracellular coccidian parasites that infect the microvilli epithelial lining of the digestive tract and rarely the respiratory tract. Cryptosporidium parvum, which is the most common member of the genus, is the causative agent of Cryptosporidiosis. These organisms are in the same order as the Plasmodium (the Malaria parasite), but the developmental life cycle, transmissibility and diseases are very different. Although recognized and identified as a parasite for a long time, the first cases of human Cryptosporidiosis were reported in 1976. Cryptosporidium is capable of infecting various types of farm and domestic animals as well. This parasite is recognized worldwide as a causative agent of diarrhea. The source of human infections is thought to be through zoonotic transmission (mainly calves, but other animals such as rodents, puppies, and kittens) and through person-to-person contact. However, this mode of transmission alone does not account for the wide-spread transmission, and epidemiological studies have shown that Cryptosporidium parvum is a water-borne pathogen. In the Spring of 1993, a massive outbreak of Cryptosporidiosis occurred in the Metropolitan Milwaukee area afflicting an estimated 400,000 persons (the largest single documented outbreak of an infectious disease in North America). This outbreak was linked to the city's water supply.
The most common clinical indications of Cryptosporidium infections are frequent watery diarrhea and low grade fever. Other symptoms include: cramps, nausea, vomiting and weight loss. Both the duration of symptoms and severity of disease and outcome vary according to age and immune status of the patient.
In immuno-competent persons, the infection causes watery diarrhea of a median duration of 10 days (range 1-20), with varying degrees of occurrence of other symptoms. The infection is considered self-limiting, but in children and infants, it has been associated with causing malnutrition, severe morbidity and spread to cause large outbreaks.
In immuno-compromized persons, the duration, severity and outcome of disease depend on the severity and cause of immune deficiency. For example, in certain patients with AIDS, infections with Cryptosporidium causes severe, prolonged diarrheal illness with malnutrition and dehydration and may be a major factor leading to death due to excessive loss of water. Involvement of biliary and respiratory trees can also occur and complicates disease further. For other patients (e.g., persons on steroid therapy), the infection may be cleared upon termination of the immuno-suppressive agent.
The infection begins with the organism colonizing the ileum and jejunum causing impaired digestion and malabsorption due to parasite-induced damage to the villi. The secretory (Cholera-like) diarrhea suggests a toxin-mediated outpouring of fluids into the gut, but no toxins have been documented as yet. Cryptosporidium is associated with diarrheal illness in all areas of the world. It is estimated that the overall prevalence of Cryptosporidium in individuals with diarrhea is 2-2.5% for persons living in industrialized countries and 7-8.5% for persons living in developing countries. The overall prevalence rate reported in various studies in North America has ranged between 0.6%-4.3% (2% in AIDS patients).
There is currently no standard effective therapy for Cryptosporidiosis. As a diarrheal disease, therapy for Cryptosporidiosis relies on relieving symptoms as well as specific therapy, with anti-cryptosporidial drugs and hyper immune globulin. Current treatment in normal hosts is symptomatic. Fluid and electrolyte replacement is the primary importance in management. Non-specific anti-diarrheal agents such as Kaopectate, Loperamide (Immodium), Phenoxylate (Lomotil), and Pepto-Bismol are not consistently effective. To date, specific treatment of Cryptosporidium immune-deficient persons has been also unsuccessful. A number of drugs have been evaluated using animal models and none have shown good promise in eliminating the infection. Immunotherapy using Bovine dialyzable Leukocyte extracts and Passive lacteal immunity using antibodies in hyper immune Bovine colustrum have shown different results.
Several treatment modalities have been tried either in individual cases or in limited scale controlled studies, and have shown various degrees of success. Examples include: Diloxamide furoate and furazolidone (DNA damaging agents Nitrofuran analog anti-Giardia drug), Quinine plus Clindamycin, oral Spiramycin (a macrolide), alpha-difluoromethylornithine (active against other parasites and P. carinii), and Interleukin-2.
As noted above, effective treatment of Cryptosporidium infections is lacking. Generally, this has not been a major problem in healthy persons because diarrhea usually lasts for less than 20 days and clinical symptoms usually are resolved spontaneously. However, recent resurgence of large outbreaks have demonstrated an association between this infection and malnutrition, and therapy may be warranted. If a safe and effective therapy were available, most clinicians would tend to treat the infection, regardless of the immune status of the patient (this would be done to prevent progression to more severe disease, and to block transmission to other susceptible hosts). Since most immuno-compromized patients often develop a prolonged, life threatening infection, an effective therapy is needed for this particular patient population.
Helicobacter pylori causes chronic gastritis in humans and has been implicated as a pathogenic factor in gastric and duodenal ulcers, gastric carcinoma and non-ulcer dyspepsia. These are important diseases because of their prevalence, their impact on morbidity and mortality and because of their cost to the health system. Diseases associated with H. pylori infection are primarily chronic conditions with multifactorial causes, although products which successfully eradicate H. pylori infection should greatly reduce the incidence and prevalence of these diseases.
Worldwide sales of anti-ulcer drugs exceed $6.5 billion. H. pylori-associated diseases generate enormous amounts of revenue for pharmaceutical firms. The market for GI drugs is currently dominated by histamine H2 receptor antagonists. Consequently, there exists a medical need for novel anti-H. pylori agents. The major thrust amongst pharmaceutical firms has been to evaluate existing products. Only two new antibiotics are in development: Abbott's Biaxin which was recently approved for the treatment of H. pylori infections and Azithromycin, a related macrolide from Pfizer, has shown promise.
From an economic perspective, antibiotics represent the treatment of choice for therapy of duodenal ulcers. Compared with other options (intermittent or maintenance therapy with H2 antagonists, highly selective vagatomy), antibiotics are relatively cheap and provide the least time spent with an active ulcer.
The major obstacle to successful eradication of H. pylori is gaining access to the organism. H. pylori is relatively easy to kill in vitro. It is susceptible to acids, bismuth and many antibiotics, but none of these are effective when used for monotherapy in vivo. Eradication rates with monotherapies have rarely exceeded 10%. Successful treatment of H. pylori requires an understanding of the physiology of the gastrointestinal sites where the infection resides and the pharmacokinetic nature of the agents used. The bacteria reside under and within gastric mucus, in gastric glands and intracellular spaces, and in the duodenal mucosa. These diverse sites mean that effective delivery of antimicrobial agents by either local or systemic is difficult to achieve. Levels of amoxycillin, bismuth and imipenem/cilastatin in the human gastric mucosa after oral administration have all been shown to exceed the in vitro MIC for the organism, although none of these agents have demonstrated efficacy in vivo. Reasons for this include failure of the drugs to penetrate into all sites of H. pylori colonization and inability to maintain adequate bactericidal levels in the mucosa. Failure of drugs like clindamycin, erythromycin and the quinolones may be due to the effect of intragastric pH. In addition, development of resistance occurs rapidly in H. pylori and has been documented for fluoroquinolones, nitroimidazoles and macrolides.
A need continues in the art, however, for improved agents that exhibit antimicrobial activity against pathogenic mycobacteria, Clostridium, Cryptosporidium and Helicobacter, and more particularly for agents and their derivatives that may be highly useful in the treatment of MDRTB.